An optical transmitter may use a variety of modulation techniques to modulate an optical signal to carry information. Polarization multiplexing (“PM”) is a modulation technique where two independent electrical data signals are modulated onto an optical carrier wave having orthogonal polarizations (e.g., an X polarization and a Y polarization) so that the overall data throughput is doubled without doubling the spectral bandwidth. Quadrature amplitude modulation (“QAM”) is a modulation technique where two or more binary or multi-level electrical data signals are modulated, via an in-phase, or “I” channel, and a quadrature (90 degree) phase, or “Q” channel, onto a single optical carrier wave such that both the amplitude and the phase of the optical carrier wave are modulated with data to enhance the efficiency of the spectral occupancy.
In M-ary QAM (“mQAM” or “M-QAM”), one symbol may represent multiple bits to enhance the spectral efficiency. For example, modulation using 16-QAM can carry 4 bits per symbol, modulation using 64-QAM can carry 6 bits per symbol, etc. Generally speaking, a signal modulated using M-QAM can carry log2(M) bits per symbol. Other modulation formats include binary phase-shift keying (“BPSK”), which is functionally equivalent to 2-QAM modulation, and quadrature phase-shift keying (“QPSK”), which is sometimes referred to as 4-QAM. A semiconductor-based modulator, such as a modulator constructed of indium phosphide, may be used in the generation of M-QAM or PM-M-QAM optical signals.